The present invention concerns a novel cardiac pacing system, and more particularly, a cardiac pacing system which is responsive to physiological requirements of the body.
Presently, activity-type cardiac pulse generators, namely, cardiac pacers, fall into three major categories.
The first category comprises those cardiac pacers whose output is generally controlled by the patient's or wearer's own cardiac rate. A base or minimum rate is programmed by the physician. When the patient's cardiac rate (either atrial or ventricular or both) decreases below this minimum programmed rate, the output of the cardiac pacer is suppressed. This is not a "true" activity pacer because only the cardiac electrical events dictate the rates or the escape interval of the implanted cardiac pacer. The "escape interval" is the time, usually in milliseconds, between successive output pulses of the cardiac pulse generator. An increase in the escape interval results in a decrease in rate. Conversely, a decrease in escape interval is an increase in rate.
The second category includes those cardiac pacers which have an activity sensor, the output of which determines the escape interval of the cardiac pulse generator. Increased physical activity of the patient will activate the sensor. The output of the sensor will provide a decrease in the escape interval or an increase in the rate of the stimulating of the cardiac muscle. As the patient's physical activity decreases, the escape interval will increase to some programmed level. Examples of this category are disclosed in U.S. Pat. Nos. 4,140,132 and 4,428,378.
The third category includes cardiac pacemakers which, as a part of the system, incorporate a physiological sensor such as one to measure the oxygen level of the pH level of the blood. Control circuitry which may include a microprocessor based on input from the physiological sensor dictates the escape interval changes. In this configuration, the physiological sensor requires current from the batteries to remain "on" at all times to monitor the physiological parameter chosen for measurement. This imposes a continuous drain on the batteries used to power the implantable pulse generator in addition the electrical energy required for sensing cardiac electrical events and emitting pulses to stimulate the cardiac muscle. An example of this category is disclosed in U.S. Pat. No. 4,467,807.
However, such physiological sensors are considered desirable for true physiological pacing or "closed loop" pacing. While such cardiac pacers have the advantage of being responsive to physiological requirements of the body (for example, the need for an increase in oxygen, the need to decrease carbon dioxide, the need to change pH level, etc.), they have the disadvantage of placing an increased energy demand on the batteries.
It is, therefore, an object of the present invention to provide a physiological sensitive cardiac pacer which does not have the disadvantage of placing a substantially increased energy demand on the batteries.
Another object of the present invention is to provide a physiologically sensitive cardiac pacer that is relatively simple in construction and easy to manufacture.
Other objects and advantages of the present invention will become apparent as the description proceeds.